Switching system reliability

ABSTRACT

A switching system is disclosed in which inter-switch unit trunks interconnect the switch units of a distributed switching system and when the central switch, which normally advances connections between the switch units is unavailable, connections between switch units are advanced by control units of the switch units over the inter-switch unit trunks. The inter-switch unit trunks comprise a connection arrangement which is an alternative to the central switch. In one embodiment, the control unit of a first switch unit detects the unavailability of the central switch to advance connections and in response to a request for connection to destination line or trunk at a second switch unit transmits signaling information on a selected inter-switch unit trunk to the second switch unit. The second switch unit responds to the signaling information by completing a connection from the selected inter-switch unit trunk to the destination. In a second embodiment, an alternative path between an origination and destination switch unit is completed, via a first and second inter-switch unit trunk, which are serially connected by a selected path through an intermediate switch unit. The use of inter-switch unit trunks as an alternative to the central switch avoids possible subscriber isolation during a central office catastrophe, such as a major fire.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to the following applications:

M. T. Ardon application serial number 356,807 filed May 24,1989"Improvement in Switching System Reliability", and

M. T. Ardon application serial number 356,802 filed May 24, 1989,"Improvement in Switching System Reliability".

The related applications are filed concurrently herewith and areassigned to the assignee of the present invention.

TECHNICAL FIELD

This invention relates to the communication of customer information in atelecommunication switching system and particularly to arrangements forconnecting portions of the switching system via trunks when a centralstage switch arrangement is unavailable for connections.

BACKGROUND OF THE INVENTION

Local switching systems process originating and terminating callsbetween customer lines connected to the switching system and trunks toother switching systems. Since most customer lines are connected only toa single local switching system, the availability of telecommunicationservice to a customer is dependent upon the continued operability of thelocal switching system.

Such systems are designed and constructed to provide continuing reliableservice in the presence of faults. The reliability is sometimes achievedby providing identical duplicates of equipment in the more criticalparts of the system. When a failing part is detected, the duplicate isplaced in service and the failing part is romoved. With care, thesubstitution of duplicate for failing parts can occur without the lossof service to customers.

A distributed switching system is one comprised of a plurality of switchunits which interface customer lines and trunks on a peripheral side ofthe unit and which interface a central switch arrangement on the otherside of the unit. Calls between customers and/or trunks connected todifferent switch units are completed through the central switch. Aduplicated distributed switching system comprises a plurality ofduplicated switching units which are switchably connected by aduplicated pair of central switches. In the presence of faults, aduplicate switching unit is substituted for a failing one and aduplicate central switch is substituted when a central switch fails.This arrangement provides excellent service of systems which undergonormal failures in only one unit of the duplicated pairs.

A disaster, such as a fire in a switching office, may cause catastrophicfailures which are not normal and result in the loss of both units of aduplicated pair. With no duplicate to substitute for a failing unit thesystem may not be able to complete calls. For example, the failure ofboth units of the central switch will isolate the customers connected toone switch unit from the lines and trunks connected to all of the otherswitch units of the system. Not only is it impossible to reach a largenumber of the customers on the same switching system, a customer'saccess to other switching systems normally reached via trunks at otherswitching units is also lost. Such isolation of customers is asubstantial service limitation which is compounded by the fact thatdamage repair after a disaster may take an extended period of time.

Although the subscriber isolation problem is raised in the context ofdisaster which destroys the central switch, the substantial isolation ofcustomers can occur in less catastrophic ways. The switch units of adistributed switching system are connected to the central switch bycommunication links. The complete failure of the communication linksbetween a switch unit and the central switch can isolate the customersof the switching unit served by those links as much as the completecentral switch failure.

A need therefore exists in the art for arrangements which avoid customerisolation by providing switching unit-to-switching unit connections whenthe central switching arrangement of a distributed switching system,which normally connects switching units, is unavailable for connectionsfrom one switch unit to another.

SUMMARY OF THE INVENTION

This need is met in accordance with the invention in which inter-switchunit trunks interconnect the switch units of a distributed switchingsystem and when the central switch, which normally advances connectionsbetween switching units is unavailable, connections between switchingunits are advanced by control units of the switch units overinter-switch unit trunks. The use of inter-switch unit trunks as thealternative to the central switch avoids subscriber isolation during acatastrophe such as a major central office fire.

Advantageously, when an inter-switch unit trunks is used to advanceconnections it is also used to convery the switch unit-to-switch unitsignaling needed to coordinate the establishment of the alternativepath. Such signaling includes the information defining a calldestination. A switch unit receiving information at a line or trunkdefining a call destination at another switch unit forwards thesignaling information to the switch unit of the call destination over aselected inter-switch unit trunk to advance the call. The switch unitreceiving the original call signaling information connects the line ortrunk on which the signaling is received to the inter-switch unit trunkand the destination switch unit, in response to the inter-switch unittrunk signaling, connects the call destination to the inter-switch unittrunk.

When more than two switch units are used to construct the distributedswitching system, the number of inter-switch unit trunks required toprovide alternative paths can be reduced when a switch unit is used toselectively interconnect inter-switch unit trunks. In an illustrativeembodiment, an alternative path is established between a first andsecond switch unit via two inter-switch unit trunks serially connectedby an intervening third switch unit. The first switch unit selects afirst inter-switch unit trunk to the intervening switch unit andtransmits signaling information thereto. The intervening switch unitselects a second inter-switch unit trunk, in response to the signalinginformation, and advances the connection to the second (destination)switch unit over the second inter-switch unit trunk. The interveningswitch unit then completes a connection between the first inter-switchunit trunk and the second inter-switch unit trunk to interconnect thefirst and second switch units.

A method for providing alternative paths between the switch units of adistributed switching system is also described in the embodiments. Forexample, a first switch unit receives call signaling informationdefining a call destination appearance and makes a determination, basedon previously stored status information, that central connectionarrangement is unavailable for the connection. The first switch unitidentifies, from the call signaling information, a first inter-switchunit trunk to a second switch unit and then call signaling informationidentifying the call destination is transmitted on the firstinter-switch unit trunk to the second switch unit. The latter identifiesthe call destination from the received signaling information andcompletes connection between the destination appearance and theinter-switch unit trunk. Alternatively, the second switch unitidentifies, from the call signaling information on the first switch unittrunk, a second inter-switch unit trunk to a third switch unit.Signaling information is then transmitted on the second inter-switchunit trunk to advance the connection.

The switch units each comprise a control unit which cooperates with acentral controller associated with the central switch to determine theavailability of the central switch for advancing connections. A controlunit which fails to receive messages from the central control for apredetermined period of time recognizes central switch unavailabilityand stores central switch unavailability status information in anassociated memory. The control unit uses the stored status informationto determine when inter-switch unit trunks are to be used to advanceconnections.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a telecommunication switching system;

FIG. 2 is block diagram of a time-slot interchange unit employed in FIG.1;

FIG. 3 represents a digital data word;

FIGS. 4 and 5 represent arrangements for interconnecting switch modulesusing inter-switch module trunks; and

FIG. 6 is a flow diagram of a call completion process.

DETAILED DESCRIPTION

The time-division switching system of FIG. 1 is used to interconnectsubscriber sets such as subscriber sets 23 through 26 and trunks such astrunks 43 through 46 and includes a time-multiplexed switch 10comprising a time-shared space division switch having 64 input terminaland 64 output terminals. Also included is a plurality of time-slotinterchange units of which representative time-slot interchange units 11and 12 are specifically shown. Each time-slot interchange unit 11 and 12includes a bidirectional time-slot interchanger and is connected to twoinput terminals and two output terminals of time-mutiplexed switch 10.In FIG. 1 time-slot interchange unit 11 is connected to twotime-multiplexed switch input terminals via time-multiplexed lines 13and 14 and to two output terminals, via time-multiplexed lines 15 and16.

In the description which follows, the input and output terminals oftime-multiplexed switch 10 are referred to as input/output ternminalpairs. This term is used since the source for data words to an inputterminal of a given input/output terminal pair is also the destinationfor data words from the output terminal of that pair. As shown in FIG.1, input/output terminal pair P1 is associated with time-multiplexedlines 13 and 15. Each time-multiplexed line 13 through 16 conveysdigital information in 125 microsecond frames each comprising 256 timeseparated channels. Accordingly, each time-slot interchange unittransmits and receives up to 512 channels of digital information duringeach 125 microsecond frame.

Each time-slot interchange unit is uniquely associated with a controlunit of which control unit 17 is associated with time-slot interchangeunit 11, and control unit 18 is associated with time-slot interchangeunit 12. Additionally, each time-slot interchange unit is connected viaindividual time-multiplexed lines to a plurality of peripheral units ofwhich line units 19 through 22 and trunk units 39 through 42 are shownin FIG. 1. A time-slot interchange unit, its associated control unit andperipheral units are collectively referred to herein as a switchingmodule which is represented in FIG. 1 as switching modules 201 and 202.Line unit 19 and 20 and trunk unit 39 and 40 are connected to time-slotinterchange unit 11 of switching module 201 and line units 21 and 22 andtrunks units 41 and 42 are connected to time-slot interchange unit 12 ofswitching module 202. Each of the line units is connected to a number ofsubscriber sets of which subscriber sets 23 through 26 are shown. Theexact number of line units associated with each time-slot interchangeunit and the exact number of subscriber sets associated with each lineunit is determined by the number of subscribers to be served and thecalling rates of those subscribers. Each line unit terminates the analogloop of the well-known type from a plurality of subscriber sets, e.g.,23 through 26, and converts call information including analog speechsignals into digital data words which are transmitted to its associatedtime-slot interchange unit. Further, each line unit detects servicerequests from the subscriber sets and generates certain signalinginformation for those subscriber sets. The particular subscriber setsfrom which speech samples are taken and encoded, and the particulartime-multiplexed channels used to transmit the resulting code betweenthe line unit and its associated time-slot interchange unit aredetermined by the control unit of the associated time-slot interchangeunit.

The trunk units, e.g. 39 and 40, perform analogous functions for trunkssuch as detecting trunk seizures and sending and receiving trunksignaling to and from other systems. The trunks can be either of theanalog or digital type. One example of a digital trunk is the T1 carriersystem disclosed in the J. H. Green et. al., U.S. Pat. No. 4,059,731, onwhich 24 separate communication channel are multiplexed.

The relationship of subscriber sets, line/trunk units and time-slotinterchange units is substantially the same for each of such groups ofinterconnected inputs. Accordingly, while the description which followsrelates directly to subscriber set 23, line unit 19 and time-slotinterchange unit 11 (switching module 201), it shows the relationshipsfor all other groups of such units. Furthermore, an analogousrelationship exits between trunks, trunk units and time-slot interchangeunits. Line unit 19 scans the lines connected to each subscriber set todetect request for service. When such a request is detected, line unit19 transmits to control unit 17, a message indicating the request andthe identity of the requesting subscriber set. This message istransmitted to control unit 17 via a communication path 27. Control unit17 performs the necessary translation based on the service requested,the identity of the requesting subscriber set and the availableequipment, and transmits a message to line unit 19 via communicationpath 27 defining which of the plurality of time separated channelsbetween line unit 19 and time-slot interchange unit 11 is to be used totrasmit information from subscriber set 23 to time-slot interchange unit11. Based on this message, line unit 19 encodes the analog informationfrom subscriber set 23 into digital data words and transmits theresulting data words in the assigned channels. FIG. 3 represents thedigital data word format used in the present system which formatincludes an 8-bit data portion and a 7-bit signaling portion. Line unit19 also transmits in the signaling bit position labeled A (the A-bit) ofthe assigned channel an indication of the DC state i.e., opencircuit/closed circuit of the subscriber loop associated with subscriberset 23.

After a time separated channel between line unit 19 and time-slotinterchange unit 11 is assigned to a given subscriber set, control unit17 detects signaling information from the subscriber set by sampling theinformation transmitted in he assigned channel. Such sampling operationsare performed via a communication path 28. Control unit 17 responds tothe signaling information from the subscriber's channel, and to controlmessages from other control units, e.g., 18, and a central control unit30, by controlling the time-slot interchange function of the time-slotinterchange unit 11.

The trunks, e.g., 43 and 44 are assigned a time-separated channel to thetime-slot interchange unit at system initialization and do not requirethe assignment of a channel when trunk activity begins (origination).The trunk units, e.g., 39 and 40 of the embodiment detect trunksignaling such as trunk seizure and reflect this signaling in the A-bitof the pre-assigned channel. Inband signaling, such as MF tones, isconveyed to the time-slot interchange unit via the PCM data portion ofthe assigned channel. Control unit 17 detects trunk signaling via thecommunication path 28 in the same manner that subscriber signaling isdetected.

As previously state, each time-multiplexed line between a time-slotinterchange unit and the time-multiplexed switch 10 has 256 channelseach 125 microsecond frame. These channels are assigned numericaldesignations from 1 to 256 in sequence as they occur. This sequence ofchannels recurs so that a given channel will be available every 125microseconds. The time-slot interchange function takes the data wordsreceived from the line and trunk units and places them in channels onthe time-multiplexed line between the time-slot interchange units andthe time-multiplexed switch 10 under the control of control units 17 and18.

Time-multiplexed switch 10 operates in recurring frames of time-slotswhere each 125 microsecond frame comprises 256 time-slots. During eachtime-slot, time-multiplexed switch 10 is capable of connecting datawords received at any of its 64 input terminals to any of its 64 outputterminals in accordance with time-slot control information stored in acontrol memory 29. The configuration pattern of connections throughtime-multiplexed switch 10 repeats itself every 256 time-slots and eachtime-slot is assigned a numerical designation in sequence from 1 to 256.Accordingly. during a first time-slot TS 1 the information in a channel(1) on time-multiplexed line 13 may be switched by time-multiplexedswitch 10 to an output terminal P64 while during the next time-slot TS 2the next channel (2) on time-multiplexed line 13 may be switched to anoutput terminal P57. Time-slot control information is written intocontrol memory 29 by central control 30 which generates this controlinformation from control messages obtained from various control units,e.g., 17 and 18.

Central control 30 and control units 17 and 18 exchange control messagesutilizing selected channels called control channels of thetime-multiplexed lines, e.g., 13 through 16, between the time-slotinterchange units and the time-multiplexed switch 10. Each controlmessage comprises a plurality of control words and each control channelcan transmit one control word per frame of 256 time separated channels.The same channel of the two time-multiplexed lines associated with agiven input/output terminal pair is predefined to be a control channel.Additionally, a given channel is used as a control channel for only onepair of time-multiplexed lines. For example, if channel 1 is used as acontrol channel on time-multiplexed line 13 and the associatedtime-multiplexed line 15, no other time-multiplexed line will usechannel 1 as a control channel. During each time-slot having the samenumerical designation as a control channel, time-multiplexed switch 10connects the data word occupying that control channel to output terminalP64 and connects input terminal P64 to the output terminal associatedwith the above-mentioned control channel.

The following is an example of the embodiment when channel 1 is thecontrol channel for time-multiplexed lines 13 and 15, and channel 2 isthe control channel for time-multiplexed lines 14 and 16. Duringtime-slot TS 1 information from control memory 29 defines, among otherconnections, that the control word in channel 1 of time-multiplexed line13 is connected to output terminal P64 and that the control word inchannel 1 at input terminal P64 is connected to time-multiplexed line15. Similarly, during time-slot TS 2, information from control memory 29defines that the control word in channel 2 of time-multiplexed line 14is connected to output terminal P64 and that the control word in channel2 at input terminal P64 is connected to time-multiplexed line 16. Whenoperating in this manner, output terminal P64 receives fromtime-multiplexed switch 10 all control words in a channel having thesame numerial designation in which they were transmitted to thetime-multiplexed switch. Further, each control channel is connected toreceive control words from input terminal P64 during the time-slothaving the same numerical designation as their associated controlchannel. Control words switched to output terminal P64 are transmittedto a control distribution unit 31 which temporarily stores them in alocation associated with that control channel. The association ofcontrol channels with storage locations in control distribution unit 31identifies the source of the information stored.

Each control message from a time-slot interchange unit comprises adestination portion and a signaling information portion. The destinationportion uniquely defines the expected destination of the signalingportion of the control message. Control distribution unit 31 interpretsthe destination portion of each control message to determine the properdestination for the control message and retransmits the message to inputterminal P64 of time-multiplexed switch 10 in a channel having the samenumerical designation as the control channel associated with thedestination unit.

When operating as above-described, time-interchange unit 11 transmitscontrol messages to time-slot interchange unit 12 by transmittingcontrol words during its recurring control channel to form a controlmessage having a destination portion identifying time-slot interchangeunit 12. Control distribution unit 31 accumulates the control words,interprets the destination portion, and retransmits the message to inputterminal P64 during the channel having the same numerical designation asthe control channel associated with time-slot interchange unit 12.

A control message can also be transmitted to central control 30 bydefining central control 30 in the destination portion of the controlmessage. When this occurs, control distribution unit 31 transmits themessage to central control 30 via a communication link 32 rather thanreturning it to time-multiplexed switch 10. Similarly, a massage may betransmitted from central control 30 to one of the time-slot interchangeunits by transmitting to the control distribution unit 31 a controlmessage having a destination portion defining the particular time-slotinterchange unit. This transmission is also accomplished utilizingcommunication link 32. The operation of the system shown in FIG. 1 isdescribed in detail in the above-cited Beuscher et. al. U.S. Pat. No.4,322,843.

The control messages exchanged among control units, e.g., 17 and 18 andthe central control 30 are used to advance call completion and todetermine the status of the switching system and its parts. Normal callprocessing involves the receipt of call signaling information from anappearance, e.g., line unit 19 at one switch module, the trasmission ofa control message from the receiving switch module to central control 30defining the incoming call and the transmission of control messages tothe receiving and call destination switch modules defining the paththrough the time-multiplex switch 10 to be used for call completion. Theinvolved switching modules then connect the appropriate lines or trunksto the defined time-multiplex switch 10 path. The exchange of callcompletion control message coordinates the operation of the involvedswitching modules and the central control 30 and relies on a decision bythe central control for the time-multiplex switch 10 path to be used.

Due to the internative nature of normal call completion, knowledge ofthe status of the control units and the central 30 is important to eachcontrol unit. Each control unit, e.g., 17 and 18 periodically transmitsa communication check control message to the central control 30 whichreplies with a predictable response message to the source control unit.When a control unit does not receive appropriate response messages fromcentral control 30 it assumes a failure of the central control or thetime-multiplex switch communication links thereto and changes its statusto the stand alone mode. In the stand alone mode, a control unit reactsto input stimuli without assistance from the central control 30 andwithout paths though the time-multiplex switch 10. Each control unitcomprises a memory 57 (FIG. 2) which includes a location that is markedto indicate stand alone status of that control unit.

Central control 30 expects communication check messages from eachcontrol unit, e.g., 17 at a predetermined periodic rate. Central control30 detects when such messages are not properly received, marks thedelinquent control unit as unavailable in the central control memory(not shown) and transmits unavailability control messages to theremaining control units. The control units respond to the unavailabilitynotice from central control 30 by marking the unavailable control unitas in the stand by mode in their individual control unit memory 57 (FIG.2).

Memory 57 (FIG. 2) of each control unit, e.g., 17 stores the program forthe control of its associated control unit and data regarding thefunction of the control unit, its associated time-slot interchange unitand its associated subscribers and trunks. The data stored in memory 57includes translation tables for translating calling information intoinformation identifying the routing for calls and information definingthe status of all switching modules, e.g., 201 and 202. The mainprocessing entity of control unit 17 is a processor 66 which operates inresponse to instructions data stored in memory 57. Control unit 17includes a control interface circuit 56 which receives instructions fromprocessor 66 via a bus 59 and in response thereto, communicates with theperipheral units, e.g., line units 19 and 20 and trunk units 39 and 40,via communication path 27. A DMA unit 58 is also included in controlunit 17 and is used in conjunction with an interface unit 69 to transmitand receive the control messages on time-multiplexed lines 13 through 16to and from the time-multiplex switch 10.

Each of the line and trunk units transmits recurring frames ofinformation each comprising 32 channels of 16 bits each. Thisinformation is transmitted to a multiplex unit 60 (FIG. 2) withintime-slot interchange unit 11. Multiplex unit 60 receives the outputsignals from the peripheral units which signals are selectivelytransmitted on an output time-multiplexed line 62 having 512 channelsfor each 125 microsecond frame. The connection of peripheral unitchannels to the channels of time-multiplex line 62 by multiplex unit 60is controlled by information stored in a control RAM 71 which iscyclically accessed at the time-multiplexed line 62 rate. Controlinformation is written into control RAM 71 by processor 66 at the timeof system initialization so that processor 66 is aware of the particularperipheral unit channel occupying each channel on time-multiplex line62. Similarly, a demultiplex circuit 61 receives 512 channels of 16 bitseach on a time-multiplexed line 63 which channels are distributed, in amanner determined by information stored in control RAM 71, to theperipheral unit channels connected to the line and trunk units, e.g., 19and 40.

The information transmitted in a given channel on time-multiplexed line62 is conveyed by a gate 52 and a path 62' to a receive time-slotinterchanger 50 where it is stored in a memory location uniquelyassociated with that given channel. The particular memory location ofreceive time-slot interchange 50 into which a given data word is storedis defined by time-slot designation signals generated by time-slotcounter 54. Time-slot counter 54 generates a recurring sequence of 512time-slot designations at the rate of one time-slot designation pertime-slot. The particular time-slot designation generated during thetime-slot in which a given data word is received defines the memorylocation within receive time-slot interchanger 50 which is to store thatdata word. Data words are also read from receive time-slot interchange50 at the rate of one data word per time-slot. The memory address of thedata word to be read from receive time-slot interchanger 50 during agiven time-slot is obtained by reading control RAM 55. Control RAM 55 isread once per time-slot at an address defined by the time-slotdesignation from time-slot counter 54 and the quantity so read istransmitted to receive time-slot interchanger 50 as the read address forthat time-slot.

Data words read from receive time-slot interchanger 50 are transmittedto time-multiplexed switch 10 via a time-multiplexed line 68, a gate 8,a time-multiplexed line 68' and an interface unit 69. Data words fromtime-multiplexed switch 10 are received by time-slot interchange unit 11by interface unit 69, and are conveyed via time-multiplexed line 70', agate 9 and a time-multiplexed line 70 to transmit time-slot interchanger53. For calls among the line and truck units connected to time-slotinterchange unit 11, control RAM 55 effects the operation of gates 8 and9 such that data words transmitted by receive time-slot interchanger 50on time-multiplexed line 68 are conveyed via gates 8 and 9 andtime-multiplexed line 70 to transmit time-slot interchanger 53.

Transmit time-slot interchanger 53 stores the incoming data words in alocation defined by an address form control RAM 55. Data words are readfrom transmit time-slot interchanger 53 at the address defined by thetime-slot counter 54. Data words so read are transmitted ontime-multiplexed line 63 for transmission to the peripheral units, e.g.,line unit 19. It should be noted that control RAM 55 may be implementedas a number of control memories, each associated with a particularcircuit, e.g., transmit time-slot interchanger 53. The particularconfiguration of control memories is not important to the presentdescription and may vary depending on timing and circuity requirementswithin the time-slot interchanger unit 11. The general principles oftime-slot interchanger as performed by the receive time-slotinterchanger 50, the control RAM 55, the time-slot counter 54 and thetransmit time-slot interchanger 53 are well known in the art and notdescribed in greater detail herein. One arrangement for reading andwriting data words in time-slot memories is described in detail in U.S.Pat. No. 4,035,584, J. W. Lurtz.

Line and trunk scanning and signaling are provided by the control unit17 which includes a signal processor 65 and a digital service unit 67.Signal processor 65 reduces the real time load requirement of processor66 by receiving and analyzing the signaling portion (bits A through G,FIG. 3) of each data word received on line 62' from line and trunk unitsand any transmitting signaling bits to the line and trunk units.Processor 66 reads the status of the each incoming channel ontime-multiplex line 62' from signal processor 65. This status, asattributed by processor 66 to the line or trunk connected to a channel,shows line or trunk status for call processing purposes. Signalprocessor 65 also transmits signaling information. Signaling bits arepassed from the processor 66 to signal processor 65 with indications oftheir destination channel on time-multiplex line 63. Signal processor 65then controls the signaling bits A through G of the destination channelsvia the path 64 to transmit signaling information to the line and trunkunits. Digital service unit 67 receives the PCM data portion of eachdata word received by line 62' from the line and trunk units to detectPCM tone signals from subscribers and trunks. The processor 66periodically reads the received signaling information from the digitalservices unit 67 to detect line and trunk signaling. Digital unit 67 isalso used to transmit tones and signals in PCM format via a gate 51 tosubscribers and trunks and via a gate 52 to time-multiplex switch 10.Definitions of the tones and signals are sent from processor 66 todigital service unit 67 which places signals in appropriate channels.

The primary mode of control information exchange comprises thetransmission of control messages from a source time-slot interchangeunit through the time-multiplexed switch 10 and the control distributionunit 31 and back to the destination time-slot interchange unit. Asecondary mode of communication is also used whereby control informationwith regard to a given call is transmitted from the source time-slotinterchange unit to the destination time-slot interchanger unit via thetime-multiplexed switch 10 utilizing the time-slot assigned for thatcall. The E-bit position of the data word in an active call time-slot isused for the secondary mode communication. However, it can be seen thatany or all of the signaling bits could used in this secondarycommunication mode. The E-bit serves the dual purpose of signalacknowledgement during the establishment of a path through thetime-multiplexed switch 10 and continuing path continuity check duringthe connection. The operation of E-bit accumulator 48 and E-bit checkcircuit 192, which communicates with processor 66 via conductors 193,194, and 195 in performing these dual purposes is described in detail inthe above-cited Beuscher, et. al., U.S. Pat. No. 4,322,843.

In the system described above, with regard to normal switching systemoperation, a switching module collects signaling information fromsubscriber sets, e.g., 23 and/or trunks, e.g., 44 and in cooperationwith the central control 30 and other switching modules completescommunication paths to the other subscriber sets and trunks. When thecall signaling information received by one switching module defines asubscriber set or trunk connected to another switching module, a paththrough the time-multiplex switch 10 is located and established by thecentral control 30 to complete the connection. When received callsignaling information defines a line or trunk on the same switchingmodule a communication path is completed within the switching module viagate 8 and 9 (FIG. 2).

The above-described system provides telecommunication services amongperipheral appearances (lines and trunks) connected to the sameswitching module and provides service among lines and trunk appearanceson different switch modules via connections through time-multiplexswitch 10. Should a switch module lose contact with the time-multiplexswitch 10 by a failure of its time-multiplexed links, e.g., 13 through16, a time-mutiplex switch 10 failure, a control distribution unit 31failure or a central control 30 failure, connection can be provided onlyamong the lines and trunks connected to the same switch module. In somesituations this limited service is acceptable, but in most situationsthe inability to connect lines and trunks on different switch modules isa severe service limitation.

FIGS. 4 and 5 represent improved systems and new methods of operationwhich advantageously use trunks connected to the peripheral side ofswitch units to provide alternative paths between switching modules whenthe normal paths through time-multiplex switch 10 are unavailable. FIG.6 represents a new call processing routine which is performed to takeadvantage of the arrangements of FIGS. 4 and 5.

FIG. 4 represents a time division switching system of the type.disclosed in FIG. 1 and components having the same reference numeral aslike components in FIG. 1 are the same. The system shown in FIG. 4comprises three representative switch modules 200 through 202. Theseswitch modules are interconnected by bidirectional inter-switchingmodule trunks 100, 101 and 102. Trunk 100 connects trunk unit 42 ofswitch module 202 to trunk unit 40 of switch module 201, trunk 101connects trunk unit 41 of switch module 202 to trunk unit 103 of switchmodule 200 and trunk 102 connects trunk unit 39 of switch module 201 totrunk unit 104 of switch module 200.

As previously discussed, each switching module stores in its memory 57an indication of its status and that of all other switching modulesregarding stand alone operation. A given switching module usesinter-switching module trunks, e.g., 100 through 102 for routing when itis in the stand alone mode or when it has been notified by the centralcontrol 30 that the destination switching module for an incoming call isin the stand alone mode. For the following example, it is assumed thattime-multiplex switch 10 has failed and is incapable of providinginter-switching module connections. The failure of the control messagecommunication link provided by time-multiplex switch 10 is detected bythe switching module 200 through 202, which begin the stand alone modeof operation. In the stand alone mode, the switching module controlunits, e.g. 18 no longer send messages to other control units or thecentral control 30 and take on the responsibility for seeking thedestination for each call received at that switching module. Infurtherance of this mode of operation, each control unit, e.g., 18maintains a translation table which identifies the switching moduleassociated with each subscriber set directory number of the switchingsystem and the identities of one or inter-switching module trunks to beused to connect to other subscriber sets.

In the description of FIGS. 1 through 3 a normal call processing routinefor finding connections between switching modules through thetime-multiplex switch 10 was followed. When no time-multiplex switchconnection was available, a requested call between switching moduleswould fail. FIG. 6 is a flow diagram representing a new call processingroutine which is employed to connect switching modules via inter-switchunit trunks connected among the peripheral trunk units of the switchingmodules. The switching system of FIG. 4 operates in accordance with thecall processing routine of FIG. 6 which is described in terms of anexemplary call from subscriber set 26 of switching module 202 tosubscriber set 24 of switching module 201.

Control unit 18 (block 301, FIG. 6), by means described with regard toFIGS. 1 through 3, detects when subscriber set 26 goes off-hook andbegins to collect the dialed digits. In accordance with the presentexample, the dialed digits define subscriber set 24 of switch module201. When all digits are collected (block 302) the call processingroutine proceeds to block 303 where the switching module status ischecked and a decision is made to use either normal time-multiplexswitch 10 routing or the altenative inter-switch module trunk routing.The status information stored in memory 57 (FIG. 2) is read to make thisdecision. When, as in the present example, a switch module is in thestand alone mode, normal pathing through the time-mutiplex switch 10 isnot available and the routing process continues from block 303 via path304 to block 305. In block 305 the control unit 18 consults itstranslation table in memory 57 (FIG. 2) and determines that trunk 100 isavailable to switching module 201 which is connected to the dialedsubscriber set 24. Accordingly, control unit 18 seizes trunk 100 andproceeds to send a representation of the dialed digits on trunk 100 inresponse to the appropriate acknowledgement signals from switchingmodule 201 (block 306). Switching module 202 finally completes path 205between subscriber set 26 and trunk 100 (block 307).

With some overlap of operation with switch module 202, switch module 201starts a call processing program as shown in FIG. 6 in response to theseizure of inter-switch module trunk 100 by switching module 202.Switching module 201 completes the origination with an acknowledgementsignal (block 301) and collects the digits from trunk 100 in block 302.The call processing routine in switching module 201 proceeds throughblock 303 to block 305 where control unit 17 consults its translationtable using digits received from trunk 100 and determines that theyrepresent subscriber set 24. Switching module 201 then signalssubscriber set 24 (block 306) and completes the path 206 between lineunit 20 and trunk unit 40 (block 307) upon answer at subscriber set 24.Subscriber set 24 is then connected to subscriber set 26 via the pathprovided by path 205 through time slot interchange unit 12, the trunk100 and the path 206 through time-slot interchange unit 11. This path isan alternative to the unavailable (in the present example) path throughtime-multiplex switch 10 which would have been established in block 308(FIG. 6) had trunk routing not been selected in block 303.

For complete connectability, the arrangement shown in FIG. 4 includes atleast one trunk between each pair of switch modules. This results in##EQU1## interconnecting trunks, where N represents the number ofswitching modules. As the number of switching modules increases, thenumber of interconnecting trunks becomes so large that a completelyinterconnected system of the type shown in FIG. 4 may not be desirable.

FIG. 5 shows an embodiment in which the number of interconnecting trunksis substantially reduced from ##EQU2## In FIG. 5, switching module 201is used as an intermodule trunk switch and is connected to switchingmodule 202 via a trunk 105 and to switching module 200 via trunk 106.The intermodule trunk switch provides selective trunk connection betweenswitching modules, e.g., 200 and 202.

In the example which follows, all switching modules 200 through 202 arein the stand alone mode and subscriber set 26 of switching module 202desires connection to subscriber set 108 of switching module 200. All ofthe switching modules 200 through 202 perform call processing inaccordance with the flow diagram of FIG. 6. Switching module 202 detectsthe origination by subscriber set 26 and determines from the dialeddigits that trunk 105 can be used to advance a connection towardsubscriber set 108. Switching module 202 then completes a path 207between subscriber set 26 and trunk 105 and transmits the dialed numberon inter-switch module trunk 105. Switching module 201 receives thedigits on trunk 105 and determines from its translation table thatsubscriber set 108 can be reached on trunk 106 to switching module 200.Switching module 201 then completes path 208 between trunk 105 and trunk106 and sends the number originally dialed by subscriber set 26 on trunk106. Switching module 200 receives the number on trunk 106 and by use ofits translation table identifies subscriber set 108 as the calldestination. Switching module 200 signals subscriber set 108 andcompletes a path 209 between subscriber set 108 and trunk 106, when thesubscriber at subscriber set 108 answers. A path consisting of path 209,trunk 106, path 208, trunk 105 and path 207 exists between calledsubscriber 108 and calling subscriber 26.

In the preceding examples, all of the switching modules are in the standalone mode. It is possible that some, but less than all, of theswitching modules required for connection are in stand alone mode andother switching modules still employ connection path through thetime-multiplex switch 10. When, for example, switching module 202 is inthe stand alone mode and desires a connection to a subscriber 108 onswitching module 200, a first path will be established to switchingmodule 201 using the path discussed with regard to FIG. 5 consisting ofpath 207 and trunk 105. Switching module 201 receives the digits ontrunk 105, and since it is not in the stand alone mode, completes a pathto switching module 200 via the time-multiplex switch 10 in the normalmode. That is, module 201 communicates with central control 30 andcooperates therewith to complete a path to switching mode 200 via thetime-multiplex switch 10. Switching module 200 then finishes the pathfrom time-multiplex switch 10 to subscriber set 108.

When a called subscriber is connected to a switching module which is inthe stand alone mode and the calling switching module is not in thestand alone mode, trunk routing is employed by the calling switchingmodule. In this situation, the central control 30 continues to notifyall switching modules of those switching modules which are in the standalone mode so that proper routing choices are made in block 303 of FIG.6.

We claim:
 1. A telecommunications switching system comprising:aplurality of switch units, each comprising a plurality of lineappearances and a plurality of trunk appearances, for selectivelyconnecting said appearances to a central connection means; said centralconnection means connected to said plurality of switch units andcooperative with said switch units for selectively interconnecting saidswitch units to advance connections between ones of said appearances ondifferent ones of said switch units; and an alternative arrangement forconnecting a first one of said appearances on a first one of said switchunits to a second one of said appearances on a second one of said switchunits comprising: means for connecting a predetermined one of said trunkappearances on said first switch unit to a predetermined one of saidtrunk appearances on said second switch unit; and control means foradvancing a connection between said first and second appearances viasaid connecting means when said central connection means is unavailableto interconnect said first and second switch units.
 2. The system ofclaim 1 wherein said control means comprises:means for receiving at saidfirst appearance, call signaling information defining said secondappearance; and means, responsive to said call signaling information,for transmitting on said connecting means, trunk signaling informationdefining said second appearance.
 3. The system of claim 2 wherein saidsecond switch unit comprises means responsive to said trunk signalinginformation received on said connecting means for connecting said secondappearance to said connecting means.
 4. The system of claim 3 whereinsaid first switch unit comprises means responsive to said call signalinginformation for connecting said first appearance to said connectingmeans.
 5. A telecommunications switching system comprising:a pluralityof switch units, each comprising a plurality of line appearances and aplurality of trunk appearances, for selectively connecting saidappearances to a central connection means; said central connection meansconnected to said plurality of switch units and cooperative with saidswitch units for selectively interconnecting said switch units toadvance connections between ones of said appearances on different onesof said switch units, and an alternative arrangement for connecting afirst appearance on a first one of said switch units to a secondappearance on a second one of said switch units comprising: inter-switchunit trunk means connecting one of said trunk appearances on said firstswitch unit to one of said trunk appearances on said second switch unit;means for determining that said alternative arrangement is to be usedfor connections between said first and said second switch units; firstconnection means in said first switch unit responsive to saiddetermining means and to call signaling information defining said secondappearance, received by said first switch unit for connecting said firstappearance to said inter-switch unit trunk means; and second connectionmeans in said second switch unit for connecting said inter-switch unittrunk means to said second appearance.
 6. The switching system of claim5 comprising:means in said first switch unit for transmitting trunksignaling information defining said second appearance, on saidinter-switch unit trunk means; and said second connection meanscomprises responsive to said trunk signaling information received onsaid inter-switch unit trunk means for connecting said inter-switch unittrunk means to said second appearance.
 7. A telecommunications switchingsystem comprising:a plurality of switch units each comprising aplurality of line appearances and a plurality of trunk appearances forselectively connecting said appearances to a central connection means;said central connection means connected to said plurality of switchunits and cooperative with said switch unit for selectivelyinterconnecting said switch units to advance connections between ones ofsaid appearances on different ones of said switch units; and analternative arrangements for selectively interconnecting said switchunits to advance connections between a first appearance on a first oneof said switch units and a second appearance on a second one of saidswitch units comprising: first inter-switch unit trunk means forconnecting one of said trunk appearances on said first switch unit toone of said trunk appearances on a third one of said switch units;second inter-switch unit trunk means for connecting another of saidtrunk appearances on said third switch unit to a trunk appearance onsaid second switch unit; and means in said third switch unit forconnecting said one appearance on said third switch unit to said anotherappearance on said third switch unit.
 8. The system of claim 7comprising means in said first switch unit for connecting said firstappearance to said first inter-switch unit trunk means; andmeans in saidsecond switch unit for connecting said second appearance to said secondinter-switch unit trunk means.
 9. A telecommunications switching systemcomprising:a plurality of switch units each comprising a plurality ofline appearances and a plurality of trunk appearances for selectivelyconnecting said appearances to a central connection means; said centralconnection means connected to said plurality of switch units andcooperative with said switch units for selectively interconnecting saidswitch units to advance connections between ones of said appearances ondifferent ones of said switch units: means for receiving first callsignaling information at a first appearance on a first one of saidswitch units, said first call signaling information defining a secondappearance on a second of said switch units; and an alternativearrangement for connecting a first appearance on a first one of saidswitch units with a second appearance on a second one of said switchunits comprising: first inter-switch unit trunk means for connecting oneof said trunk appearances on said first switch unit to one of said trunkappearances on a third one of said switch units; second inter-switchunit trunk means for connecting another of said trunk appearances onsaid third switch unit to a trunk appearance on said second switch unit;and connection means in said first switch unit responsive to said firstcall signaling information for connecting said first appearance to saidfirst inter-switch unit trunk means; signaling means, in said firstswitch unit, for transmitting on said first inter-switch unit trunkmeans second call signaling information defining said second appearance;connection means in said third switch unit responsive to said secondcall signaling information for connecting said first and said secondinter-switch unit trunk means; signaling means in said third switch unitfor transmitting on said second inter-switch unit trunk means third callsignaling information identifying said second appearance; and means insaid second switch unit for connecting said second inter-switch unittrunk to said second appearance.
 10. In a telecommunication switchingsystem comprising:a plurality of switch units, each comprising aplurality of line appearances and a plurality of trunk appearances, forselectively connecting said appearances to a central connection means;said central connection means connected to said plurality of switch unitand cooperative therewith for selectively interconnecting said switchunits to advance connections between ones of said appearances ondifferent ones of said switch units; and a plurality of inter-switchunit trunk means for interconnecting pairs of said trunk appearances ondifferent ones of said switch units, a method comprising: receiving at afirst appearance of a first switch unit, call signaling informationspecifying a call destination; determining that said central connectionmeans is unavailable to advance a connection to said call destination;responsive to said determining step, identifying from said callsignaling information, a first one of said inter-switch unit trunk meansconnecting said first switch unit to a second switch unit, when saidcentral connection means is unavailable; and transmitting on said firstone of said inter-switch unit trunk means first trunk signalinginformation specifying said call destination.
 11. The method of claim 10comprising:identifying from said first trunk signaling information asecond appearance on said second switch unit; connecting, at said secondswitch unit, said identified second appearance to said firstinter-switch unit trunk means; and connecting, at said first switchunit, said first inter-switch unit trunk means to said first appearance.12. The method of claim 10 comprising:identifying from said first trunksignaling information, a second one of said inter-switch unit trunkmeans connecting said second switch unit to a third switch unit; andtransmitting second trunk signaling information specifying said calldestination, on said second one of said inter-switch unit trunk means.13. The method of claim 12 comprising:identifying from said second trunksignaling information, a second appearance on said third switch unit,connecting, in said third switch unit, said second appearance to saidsecond inter-switch unit trunk means, connecting in said second switchunit, said first inter-switch unit trunk means to said secondinter-switch unit trunk means, and connecting in said first switch unit,said first inter-switch unit trunk means to said first appearance.